1. Field of the Invention
This invention relates to an improved check valve which prevents unintentional fluid leakage past the valve even at low fluid pressures. More particularly, it relates to a piston in such a check valve which supports an annular seal in such a manner that the seal bends when contacting an annular seat.
2. Description of the Prior Art
Generally speaking, check valves are well known. In this context, a check valve is taken as being any device which permits fluid flow in one direction while inhibiting fluid flow in the reverse direction. In many situations an ideal check valve would permit free flow of the fluid in the one direction while preventing all undesired fluid flow in the reverse direction. A typical check valve incorporates an enlarged chamber having a movable member (e.g. piston, ball, etc.) therein and a seat at one end of the chamber. Reverse fluid flow biases the movable member towards the seat, with the contact between the member and seat providing a seal which inhibits reverse fluid flow.
In such a check valve system, it takes a certain threshold of reverse fluid pressure to effectively bias the member into sealing disposition with the seat. Further, a small amount of reverse fluid flow can occur until the movable member is biased towards the seat. A common characteristic of such check valves is that the higher the reverse fluid pressure, the better the seal between the movable member and the seat. Many types of check valves are ineffective in preventing undesired reverse fluid flow at low fluid pressures.
Check valves or expansion devices for use in refrigerant systems are generally illustrated by U.S. Pat. Nos. 3,877,248; 3,642,030; and 3,992,898, which are incorporated herein by reference. FIG. 2 of U.S. Pat. No. 3,992,898 illustrates a check valve for use in a reversible heat pump system. In such a heat pump valve, it is desirable to permit fluid flow in either direction, with a large amount of fluid flow permitted in the first direction, and a relatively small, metered fluid flow permitted in the reverse direction. In refrigerant systems, it is desirable that check valves prevent unwanted fluid flow even at low fluid pressures. For example, viewing FIG. 2 of U.S. Pat. No. 3,992,898, even though it is desirable to permit fluid flow in a reverse direction through the central metering port 46, it is nevertheless desirable to prevent reverse fluid leakage around the perimeter of the sliding piston 45. Those well versed in refrigerant systems will appreciate that often times the reverse fluid pressure is relatively low, but nevertheless it is important to allow only the metered amount of fluid to pass the valve through the metering port.
FIG. 4 illustrates a typical check valve in a refrigerant system which is designed to prevent fluid flow in the reverse direction (upwardly viewing FIG. 4) while permitting fluid flow in a first direction (downwardly viewing FIG. 4). It will be appreciated that in heat pump type systems, a centrally located metering port might be incorporated in the piston of FIG. 4 to allow a small, metered amount of reverse fluid flow. Viewing FIG. 4, the seal which prevents reverse fluid flow is provided by the compression of the gasket G contacting the annular seat S of the check valve. That is, the reverse fluid pressure biases the piston P in the reverse (upwardly) direction whereupon the gasket G compressingly engages seat S. The gasket G is typically a teflon ring which is stretched over the piston P during assembly. Often there are minor imperfections in the thickness of the gasket G and also minor installation errors can occur during assembly. The result is slight fluid leakage in the reverse direction past the gasket until the fluid pressure is high enough to compress and deform the gasket G against the seat S.
FIG. 6 illustrates typical test results of the leakage past the check valve illustrated in FIG. 4. The abscissa of the graph of FIG. 6 represents the reverse fluid pressure applied through the valve of FIG. 4 while the ordinate of the graph represents the amount of fluid leakage past the piston P. As can be seen from FIG. 6, little leakage past the gasket G occurs at relatively high fluid differential pressures (e.g., greater than 80 psig). However, a significant amount of fluid leakage past the gasket G occurs at low fluid pressures. In refrigerant systems, such reverse fluid leakage is undesirable and leads to inefficient operation of the system.